I just created an account here and already uploaded some Scenarios and Tracks.
I noticed somebody had one available to DL with a ride already in the Scenario. I would like to do that, too, but have no idea how. Can somebody help?

when you design the scenario, build the ride as you normally would

^ Exactly as xEmmax said. The ride you are wanting to build will show up on it's own tab in the menus and then you build from there.

i've just noticed you've uploaded some rct2 scenarios, so i'm guessing this question is about rct2, so my advice is..... go get rct3! you'll never look back